Field of the Invention
The present invention relates to an image pickup apparatus, a control method therefor, and a storage medium, and in particular to a photometric control by using an image sensor.
Description of the Related Art
Generally, some of image pickup apparatuses such as digital cameras have a so called live view function. By the live view function, images according to image signals continuously read out from an image sensor are sequentially displayed on a display device such as a liquid crystal display. Accordingly, a user is able to easily check a subject.
On the other hand, when the user checks the subject via an optical viewfinder, it is advantageous to observe the subject in an actual time, and some image pickup apparatus having the live view function can switch between the optical viewfinder and the live view function.
In General, in a case where both the optical viewfinder and the live view function are installed in an image pickup apparatus, a position of an optical path dividing mirror is switched so that an optical image having passed through an image pickup optical system is guided to either of the optical viewfinder or the image sensor. Accordingly, two photometry devices and two distance measurement device are necessary, which makes a circuit configuration complicated.
In order to prevent the circuit configuration from being complicated as described above, for example, there has been proposed a technique about a photometric process and a distance measurement process for a case where an optical path dividing mirror comprised of a half mirror is set on an optical path from an image pickup optical system to an image sensor (See, Japanese Laid-Open Patent Publication (Kokai) No. 2010-16762). In this technique, the photometric process and the distance measurement process are performed by using an image signal and a focus detection signal output from the image sensor according to the optical image having passed through the optical path dividing mirror.
Further, there has been proposed a technique in which, when a light amount of an optical image having passed through a half mirror is not enough, a photometry is performed by retracting an optical path dividing mirror (See, Japanese Laid-Open Patent Publication (Kokai) No. 2010-72201).
In a case of a live view, the photometry is performed based on an image signal read out from the image sensor. In this embodiment, the photometry is performed by converting an analog image signal output from the image sensor into a digital image signal by an A/D converter. Then, a photometric luminance range according to the image signal output from the image sensor is decided based on a dynamic range characteristic of the A/D converter. For example, when an output of the A/D converter is a 14-bit signal, the photometric luminance range includes about plus/minus three degrees.
FIG. 4 is a view for use in explaining a photometric control during a live view in a conventional image pickup apparatus.
Assuming now that a subject having a brightness exceeding three steps compared to a brightness to be controlled in the live view. In this embodiment, if the photometric luminance range is limited as described earlier, it is necessary to perform a plurality of times of photometry while switching an accumulation time period (TV) of the image sensor, a readout gain (ISO), and a diaphragm (AV).
Assuming again that the photometry based on the image signal readout from the image sensor while the optical viewfinder is in use. In this embodiment, if the photometry is performed in the same manner as the case of the live view, it is necessary to control a diaphragm of the image pickup optical system.
Generally, on the other hand, an image of the optical viewfinder is preferable to be bright. Moreover, when a focus is checked using the optical viewfinder, it is difficult to check a focus if a diaphragm is small due to a deep depth of field. Therefore, it is preferable to keep the diaphragm open. This holds true for a case where a distance measurement is performed based on an image signal read out from the image pickup apparatus, and it is thus impossible to accurately perform a distance measurement with a small diaphragm. However, there is a problem that a high luminance limit of the photometry becomes lower if a diaphragm is fixed to an opened state. This problem holds true for signal processing in which a predetermined plurality of element rows (first rows) and a predetermined plurality of element rows (second rows) which does not overlap with the predetermined plurality of rows (first rows) in the image sensor are independently driven, and, for example, a live view image signal for display and image signals for the photometry and the distance measurement are output at the same time from the image sensor.